1. Field of the Invention
The present invention relates to a new bicycle rear suspension system, and in particular to a four-bar suspension system that offers improved pedaling and bump absorption performance by means of controlling the rate of chainstay lengthening through the use of a small eccentric mechanism located in close proximity to the bicycle chain line.
2. Background of the Invention
There are numerous bicycle suspension systems in use today to improve bicycle performance. A suspension system allows the rear wheel of the bike to better track the terrain resulting in improved traction while pedaling, turning, and braking. Additionally, the absorption of bump forces by the suspension system increases rider comfort. These systems range from the simple to the complex.
3. Description of Related Art
One of the simplest bicycle suspensions is the single pivot system in which a rear triangle swingarm is attached to a main front triangle at a single pivot point. With this type of suspension, the rear wheel of the bike may track the terrain but must do so while moving in a simple circular arc about the pivotal connection.
While the single pivot systems are very simple, relatively easy to design, and adequately handle small bumps, they fail to address many performance issues. Both their pedaling performance and their bump absorption capabilities are controlled by the location of the single pivotal connection between the front triangle and the rear triangle swingarm. In this type of suspension, the rear axle of the bike rotates about a single axis, and as a consequence the bike's rear suspension is either compressed or extended by the forces imparted to the pedals by the bike rider, dependent upon the specific configuration of the bike suspension. Likewise, compression and extension of the suspension can impart torque on the pedals and supply unwanted force to the rider's legs. Thus, some of the energy expended by the rider of the bike is used needlessly wasted, either compressing or extending the suspension of the system, or resisting unwanted pedal rotation imparted by the suspension.
One solution to the above problem is through the use of adjustments to the length of the chainstay during the course of compression of the bike's suspension. The chainstay length of a bicycle is the distance from the rear wheel axle to the center of the bottom bracket shell. The chainstay length of a bicycle with real wheel suspension may be variable, the length being dictated by the location of the pivotal connection between the front triangle and the rear triangle swingarm and the motion of the rear triangle swingarm. In single pivot systems there is no means for varying the rate of chainstay lengthening through the travel of the suspension. Thus, if the rate of chainstay lengthening is high, so as to decrease unwanted pedal rotation and increase the anti-squat characteristics of the suspension system, the result will be excessive chainstay lengthening at full compression and the application of excessive torque to the pedal crankarms.
In single pivot systems, the shock rate is only controlled by the single pivot point between the front triangle and the rear triangle swingarm. As a result, ineffective compromises in the suspension system's performance must always be struck. These compromises include excessive chainstay lengthening resulting in unwanted torque applied to the bicycle's pedal crankarms, extension or compression of the suspension system due to pedal forces, and extension or compression of the suspension system during acceleration and braking due to weight transfer. Additionally, as may be apparent from the above, any system that articulates the rear wheel along an arc forfeits the performance benefits associated with articulating the rear wheel along a controlled, and preferential travel path. A controlled travel path allows for, among other benefits, chainstay lengthening to occur at varying rates through the compression of the suspension.
A more complex form of bicycle suspension is a linkage system. Such systems attach the rear wheel swingarm to the main front triangle through a plurality of links to improve the performance of the suspension system by manipulating both shock rate and wheel travel path, and thus chain stay lengthening. These systems provide challenges to the suspension designer, in that there is typically a tradeoff between optimally manipulating the rear wheel travel path and controlling the change in the shock rate. In order to make some linkage suspension systems perform well, the variation in the rear wheel travel path has typically been small throughout the range of travel. Systems that vary the rear wheel travel path to a greater extent generally must also vary the shock rate to a great extent, which leads to undesirable bump absorption characteristics.
Various forms of linkage systems have been developed in an attempt to resolve the above limitations. Some linkage systems use a single pivot swingarm and a linkage to activate the shock absorber, such as the Turner 5-Spot by Turner Suspension Bicycles, Inc. in Murrieta, Calif. These systems allow for more complicated leverage ratios than a single pivot system, which results in increased tunability of the shock absorber leverage rate. However, even with this improved system, the rear wheel still moves in an arc, resulting in many of the same compromises in performance from which the single pivot suspension systems suffer.
While there is a great variety in these linkage systems, most are four-bar systems that articulate the rear wheel in a more complicated manner than is allowed with a single pivot system. Stated generally, there are two classes of four-bar linkage systems. The first class comprises systems that use a long first linkage member that places a pivot point near the rear axle. The long link systems generally have separate chainstay and seat stay assemblies that are bolted together at a pivot. An example of a long link four-bar linkage system is the Stumpjumper by Specialized Bicycle Components out of Morgan Hill, Calif., and disclosed in U.S. Pat. No. 5,899,480 to Leitner. Because this class of four bar linkage systems articulates the rear wheel in a large radius arc, the employment of an optimal amount of chainstay lengthening in the early part of the rear wheel's travel results in an unacceptable total amount of chainstay lengthening when the system is fully compressed. In order to cope with this deficiency, this style of suspension system typically employs the minimal amount of chainstay lengthening that is acceptable, so as to consequently minimize the total amount of lengthening when the system is fully compressed. Firmer shock absorber spring and damping rates are required to allow for acceptable pedaling performance.
The second class of four-bar linkage systems uses a short first linkage member where the connection between the seat stays and the chainstays is generally rigid, thereby resulting in a rear triangle wherein a swingarm is connected to two short links. Due to the short linkage members rotating with greater angular velocity when compared to the first type of four bar linkage system there is greater ability to tune the axle path of this suspension system.
The short link designs making up the second class of four-bar linkage systems can be further separated into two sub-classes. The first sub-class is one in which the two links rotate in the same direction as the suspension is compressed. U.S. Pat. No. 7,128,329 to Weagle discloses a rear wheel suspension system that falls into this first class of short link four bar linkage systems. This design utilizes anti-squat behavior to mitigate the unwanted effects of the suspension system compressing and extending due to forward acceleration when pedaling. This design suffers from the drawback that chainstay lengthening effects are minimally utilized and instead the suspension system is designed to provide an optimized anti-squat behavior for an arbitrary constant pedaling input force and gear. In practice, both the pedaling input force and gear are greatly variable and thus an assumption that they are an arbitrary constant is invalid and suboptimal.
The Rockrider by Decathlon out of Villeneuve d'Ascq, France is an example of a bicycle of the first sub-class that uses an eccentric as a link in a short link four bar linkage system. The eccentric rotates only in one direction as the suspension compresses. The distance between the pivots on the small link is significantly larger than 15 mm, and thus suffers from a weight disadvantage. This design moves the rear wheel in a very large arc, and as such suffers from the typical performance drawbacks experienced by other similar systems, such as the inability to control the chainstay lengthening effect and provide for an optimized rear wheel travel path that reduces pedaling induced compression and extension of the suspension.
In the second sub-class of short link four-bar systems the links rotate in opposite directions as the system is compressed. That is, one link rotates in a first direction and a second link rotates in a second direction. An example of the second type of short link system is the Blur by Santa Cruz Bicycles out of Santa Cruz, Calif. and disclosed in U.S. Pat. No. 6,206,397 to Klassen and Calon. These systems employ an S-shaped wheel travel path to selectively apply chainstay lengthening. The net result, however, is that as the suspension moves from its statically loaded sag point (to either a direction of further compression or a direction of reduced compression), chainstay lengthening occurs. Because chainstay lengthening occurs during extension from the suspension's statically loaded sag point, the suspension if prevented from utilizing negative travel to allow the wheel to follow drops in the terrain and thus maintain traction.
Further, due to the interaction between the linkage members of the suspension system, the suspension system once tuned to provide the described S-shaped rear wheel travel path, produces a shock rate with detrimentally high rates of change. Finally, due to the length requirements of the lower linkage member, it is impractical to utilize a small eccentric as a linkage member and benefit from the significant weight saving advantages that go along with a small eccentric linkage member.
As an example of yet another different attempt to eliminate the above common problems with bike suspensions, U.S. Pat. No. 6,099,010 to Busby discloses a bicycle having an independent equilibrium sensing suspension system for its crank assembly. This system is essentially a four bar system in which the pedal crank arm is connected to the first linkage member instead of to the front triangle. This system allows for greater rearward motion of the rear wheel without a corresponding increase in chainstay lengthening. This is accomplished by the rearward motion of the entire pedal crank assembly when the suspension is compressed. However, this motion of the pedal crank axis relative to the seat of the bicycle is perceptible to the rider, and also affects pedaling performance as the relationship between the seat and the pedals changes as the suspension is compressed.
Thus, all of the prior art suffers from a serious performance drawback: when the suspension system is configured in such a manner that the overall chainstay lengthening effect is small enough to prevent unwanted feedback to the rider's pedaling motion when the suspension system is fully compressed, the system does not offer enough chainstay lengthening effect when the suspension system is in its statically loaded sag state. None of the linkage systems currently in use or previously described drastically alter the rear wheel path to quickly move from a high rate of chainstay lengthening in the early portion of the rear wheel's motion and to the statically loaded sag point to a rate of very low chainstay lengthening after a specified point in the rear wheel's travel in close proximity to the sag point.
It is therefore a primary object of the invention to provide a suspension system offering minimal overall chainstay lengthening during full suspension compression while simultaneously offering an appropriately high level of chainstay lengthening effect when the suspension system is statically loaded.
It is a further object of the invention to provide a short link suspension system that uses a link shorter than that in conventional four bar suspension systems.
It is a further object of the invention to provide a short link suspension system comprising a link that changes its direction of rotation as the system is compressed, and more specifically comprising a short link that rotates in a first direction and then in the opposite direction as the suspension system is compressed.
It is a further object of the invention to advantageously alter the rate of chainstay lengthening in the preferential manner disclosed herein.
It is a further object of the invention to provide an optimized rear wheel travel path that allows for better pedaling performance throughout the range of suspension travel.
It is a further object of the invention to provide an optimized shock rate that advantageously provides for small changes in rate.
It is a further object of the invention to provide an optimized shock rate that advantageously changes sign more than once and articulates the rear wheel in a controlled and preferential non-arc travel path.